Most prior techniques to treat varicose veins have attempted to heat the vessel by targeting the hemoglobin in the blood and then having the heat transfer to the vessel wall. Lasers emitting wavelengths of 500 to 1100 nm have been used for this purpose from both inside the vessel and through the skin Attempts have been made to optimize the laser energy absorption by utilizing local absorption peaks of hemoglobin at 810, 940, 980 and 1064 nm. RF technology has been used to try to heat the vessel wall directly but this technique requires expensive and complicated catheters to deliver electrical energy in direct contact with the vessel wall. Other lasers at 810 nm and 1.06 um have been used in attempts to penetrate the skin and heat the vessel but they also have the disadvantage of substantial hemoglobin absorption which limits the efficiency of heat transfer to the vessel wall, or in the cases where the vessel is drained of blood prior to treatment of excessive transmission through the wall and damage to surrounding tissue. All of these prior techniques result in poor efficiency in heating the collagen in the wall and destroying the endothelial cells.
In addition, blood coagulum that accumulates on the tip of the fiber optic energy delivery device is a significant problem associated with the prior art systems. The blood coagulum will often break off of the fiber and lodge in an early section of the treated vein, where it can thrombose and travel into the patient's venous circulation system. This is a serious complication referred to as Deep Vein Thrombosis (DVT) that, in the worst cases, is fatal to the patient. The blood coagulum can also block the energy coming out of the tip of the fiber and thereby reduce the effectiveness of the treatment. The only way to detect that fiber tip coagulation is occurring is to observe the lack of vein shrinkage under ultrasound, and then to remove the fiber to examine the tip. This is a common cause of non-closures or failures of the prior art endovenous treatments. The blood coagulum is able to absorb so much laser energy that it carbonizes and may explode, causing rupture of the vein wall. Navarro claims that this carbonized blood actually turns into a hot tip or conductive heating device that can transfer energy to the vein wall. In fact, it has been shown that the carbonized blood actually prevents any direct delivery of laser energy to the vein wall until the carbon explodes, which can cause vein wall perforation.
Blood coagulum that is caused by laser or RF heat delivery can also break off and remain inside the closed vein post treatment. Research has suggested that these regions of thrombus in a treated vein may not heal in a normal way and result in the vein staying patent or open. This is considered a treatment failure. When treating hand veins, these areas of heat induced thrombus left in an otherwise completely closed vein are cosmetically unattractive and need to be surgically punctured and drained post operatively to maintain a good result. Others report that Endovenous Heat Induced Thrombus (EHIT) is an expected post-procedure outcome, although one for which several treatment strategies are suggested. See Lowell S. Kabnick et al., “Endovenous Thermal Induced Thrombosis: Classification and Suggested Treatment,” 2006 Int'l Vein Congress, Apr. 20-22, 2006. Of particular concern to physicians are instances when EHIT extends to close proximity or beyond the superficial-deep venous junction.
Baumgardner and Anderson teach the advantages of using the mid IR region of optical spectrum 1.2 to 1.8 um, to heat and shrink collagen in the dermis. The use of this wavelength region greatly reduces the occurrence of thrombus because of the lower hemoglobin absorption of these wavelengths, but since blood contains a significant amount of water it can still be heated with these water absorbing wavelengths and eventually cause a small thrombus.
The relevant references in the prior art teach the use of continuous or very long exposures of energy, such as bursts of about one second or more, up to continuous exposure. Peak power levels with these lasers run from about 10 to about 20 watts. These relatively long exposure times at these power levels are needed because the prior art laser wavelengths are not as efficiently coupled to the vessel wall and are instead absorbed in the blood or transmitted through the wall into surrounding tissue. It will be understood that methods taught in the prior art have been inefficient to such a degree that external cooling is needed on the skin surface to prevent burns. The high power and long or continuous exposure times associated with systems using high hemoglobin absorbing wavelengths have the result of creating a great deal of coagulum and thrombus. The coagulum and thrombus are effectively baked onto the tip of the fiber.
For example, Navarro et al., U.S. Pat. No. 6,398,777 issued Jun. 4, 2002, teaches a device and method of treating varicose veins that involves using laser energy whose wavelength is 500 to 1100 nm and is poorly absorbed by the vessel wall. Laser energy of wavelengths from 500 to 1100 nm will penetrate 10 to 100 mm in tissue unless stopped by an absorbing chromophore. See FIG. 5. Most of the energy used by this method passes through the vessel wall and causes damage to surrounding tissue. If sufficient blood is present in the vessel, a thrombus will form on the fiber tip that hardens and bakes on and eventually carbonizes completely over. This stops the delivery of energy to the blood (or anywhere else) until the carbon explodes, which can cause a vein wall perforation. Operative complications of this technique include bruising and extensive pain caused by transmitted energy and damage to surrounding tissue. Navarro teaches the use of peak powers of only 10 to 20 watts.
However, this technique does appear to be clinically effective because the blood that remains in the vein after compression absorbs the 500 to 1100 nm energy. 500 to 1100 nm light is absorbed in less than 1 mm in the presence of hemoglobin. See FIG. 5. This blood heats up and damages the vein wall by conduction, not by direct wall absorption as described by Navarro.
This prior art technique is also poorly controlled because the amount of residual blood in the vein can vary dramatically. During an actual procedure using 500 to 1100 nm lasers it is possible to see the effects of blood absorption of the energy. At uncontrolled intervals white flashes will be seen indicating places of higher blood concentration. The blood can boil and explode in the vessel causing occasional perforation of the vein wall and unnecessary damage to healthy tissue.
In places without residual blood the laser energy has no absorbing chromophore and will be transmitted through the wall without causing the necessary damage and shrinkage that occurs during the methods taught herein.
Navarro states that the treatment device described must be in direct “intraluminal contact with a wall of said blood vessel.” This is necessary because the 500 to 1100 nm laser cannot penetrate any significant amount of blood, even though it requires a thin layer of blood to absorb and conduct heat to the vessel wall. This is very difficult to achieve and control.
Navarro also describes the delivery of energy in long exposure bursts of one second or more with very low peak powers. This is required using the described technique because Navarro describes no means for uniformly controlling the rate of energy delivered. Navarro teaches a method of incrementally withdrawing the laser delivery fiber optic line while a low peak power laser burst is delivered. In clinical practice this is very difficult to do and results in excessive perforations and complications. The Navarro laser device does not contain any power supply or control electronics or software to produce very short, high peak power, pulsed laser outputs as described herein. Instead, the Navarro device and other prior art diode lasers depend on a mechanical footswitch device to turn the laser energy emission on and off.
Closure of the greater saphenous vein (GSV) through an endolumenal approach with radiofrequency (RF) energy has also been achieved. The RF energy is delivered very slowly in continuous mode only and has caused a significant amount of coagulation or thrombus at the cathode tip. Many procedures need to be stopped and the catheter removed and cleaned of coagulum before proceeding.
RF energy can be delivered through a specially designed endovenous electrode with microprocessor control to accomplish controlled heating of the vessel wall, causing vein shrinkage or occlusion by contraction of venous wall collagen. Heating is limited to 85 degrees Celsius avoiding boiling, vaporization and carbonization of tissues. In addition, heating the endothelial wall to 85 degrees Celsius results in heating the vein media to approximately 65 degrees Celsius which has been demonstrated to contract collagen. Electrode mediated RF vessel wall ablation is a self-limiting process. As coagulation of tissue occurs, there is a marked decrease in impedance that limits heat generation. However, this process actually encourages coagulum formation by heat thrombosis. By limiting the temperature to non-ablative and non-boiling temperatures, one is generally assured that the blood will coagulate and not boil off as taught in the present invention.
Presently available lasers to treat varicose veins endolumenally heat the vessel by targeting the hemoglobin in the blood with heat transfer to the vessel wall. Lasers emitting wavelengths of 500 to 1064 nm have been used for this purpose from both inside the vessel and through the skin. Attempts have been made to optimize the laser energy absorption by utilizing local absorption peaks of hemoglobin at 810, 940, 980 and 1064 nm. The endovenous laser treatment of the present invention allows delivery of laser energy directly into the blood vessel lumen in order to produce endothelial and vein wall damage with subsequent fibrosis. It is presumed that destruction of the GSV with laser energy is caused by thermal denaturization. The extent of thermal injury to tissue is strongly dependent on the amount and duration of heat the tissue is exposed to.
One in vitro study model has predicted that thermal gas production by laser heating of blood in a 6 mm tube results in 6 mm of thermal damage. This study used a 940-nm-diode laser with multiple 15 Joules per second pulses to treat the GSV. Histologic examination of one excised vein demonstrated thermal damage along the entire treated vein with evidence of perforations at the point of laser application described as “explosive-like” photo-disruption of the vein wall. Since a 940 nm laser beam can only penetrate 0.03 mm in blood, the formation of steam bubbles is the probable mechanism of action. See T. M. Proebstle et al., “Thermal Damage of the Inner Vein Wall During Endovenous Laser Treatment: Key Role of Energy Absorption by Intravascular Blood,” Dermatol Surg 28:7:July 2002.
Patients treated with prior art methods and devices have shown an increase in post-treatment purpura and tenderness. See, e.g., Edward G. Mackay et al., “Saphenous Vein Ablation,” Endovascular Today, March 2006, pp. 45-48. Most patients do not return to complete functional normality for 2-3 days as opposed to the 1 day down-time with RF energy-based methods for treating the GSV. Since the anesthetic and access techniques for the two procedures are identical, it is believed that non-specific perivascular thermal damage is the probable cause for this increased tenderness. In addition, recent studies suggest that low peak power laser treatment with its increased risk for vein perforation may be responsible for the increase symptoms with endovenous laser treatment (EVLT) vs. RF treatment. Slow, uncontrolled pull-back of the catheter is likely one cause for overheating and perforation of the vessel wall, as even the best surgeon may have difficulty retracting the fiber at exactly the correct speed to maintain a vessel wall heating temperature of 85 deg C. This technique prevents damage to surrounding tissue and perforation of the vessel.
Prior art endovenous lasers have typically used relatively large fiber optic catheters of about 600 μm core diameter. This size fiber is selected to maximize the amount of laser power that may be coupled into the fiber. Prior art diode lasers are difficult to focus to small spots in high power levels because each diode bar typically needs to be imaged separately into the fiber optic. This is much easier when a large core fiber with high Numerical Aperture is used.
Varicose veins are commonly treated with a fiber optic catheter and a laser to coagulate the vein wall. Thermally induced thrombus is common during endovenous ablation. Thrombus can absorb laser energy reducing efficiency of vein wall coagulation and can carbonize and explode causing vein wall perforations. Proximal thrombus can break off into the venous system and intravenous thrombus can mask non-closed segments. Pulsed mid-infrared lasers have been used since at least the 1980s for laser thrombolysis in the treatment of strokes.1 Lasers with pulse lengths from 10 nsec to 10 msec have been used to generate vapor bubbles that break up clots when they collapse. These same lasers will generate less thrombus in the presence of blood during an endovenous ablation procedure. Heating blood has been found to cause many changes in erythrocytes, such as cell shape modification, cell membrane rupturing, protein denaturation, aggregation, and blood gelation that can all possibly contribute to coagulum formation.2 Laser treatment also causes the development of fibrin through thermal alteration of fibrin complexes or proteolytic cleavage of fibrinogen3. Laser treatment of cutaneous vascular disease causes intravascular consumption of fibrin-promoting factors3. Determining the conditions for minimal coagulum formation can help to reduce rates of complications.